Nader, Simin, and their ten-year-old daughter, Termeh, form a secular, upper-middle-class family residing in a flat in Tehran. Simin harbors dreams of moving abroad with her family and has already prepared visas, while Nader is reluctant to leave his ailing father, who suffers from Alzheimer's disease, living with them. When their aspirations clash, Simin files for divorce, but the family court deems the reason inadequate and dismisses her request. Consequently, Simin returns to her parents' home, and Termeh chooses to stay with her father.
In an effort to manage the situation, Nader hires Razieh, a deeply religious woman from a less privileged suburb, to assist with his father's care during the day. Accompanying her each day is her young daughter, Somayeh. Unfortunately, Razieh soon realizes the demands of caring for Nader's father are overwhelming, especially as he has become incontinent. One afternoon, while Razieh and Somayeh are preoccupied, the elderly man escapes and meanders into the street. Razieh rushes out, navigating through traffic to retrieve him.
The following day, an unforeseen incident occurs when Nader and Termeh arrive home early and find Nader's father unconscious on the bedroom floor, tied to the bed. Razieh and Somayeh are absent at that moment. When they return, Razieh claims she had a pressing matter to attend to, but Nader accuses her of neglecting his father and stealing money meant for moving expenses, which Simin had actually used. When Razieh refuses to leave until she is compensated, Nader forcibly expels her from the apartment, causing her to stumble down a flight of stairs. Later, Nader and Simin receive troubling news that Razieh has suffered a miscarriage.
This serious turn of events prompts Nader to confront the implications of his actions. If he knew about Razieh's pregnancy and was responsible for her miscarriage, he could face murder charges. As tensions mount, the situation evolves into a cycle of accusations before a criminal judge: on one side stands Nader, supported by Simin and Termeh; on the other, Razieh and her irate husband, Hodjat. Struggling with lost dignity after being laid off from his long-held job as a cobbler, Hodjat's frustration often leads him to lash out at Nader.
Throughout the proceedings, Razieh asserts that Nader was aware of her condition, claiming he overheard a conversation between her and Termeh’s tutor about seeking medical advice. Nader vehemently denies this assertion, with the tutor backing him up. However, Termeh begins to doubt the truth of Razieh's claims, ultimately persuading her father to confess to her that he has misled the adult for their own protection; he cannot risk imprisonment. The tutor later retracts their statement, and to shield her father, Termeh tells the judge that he was oblivious to Razieh's pregnancy until she disclosed it.
Upon further examination, Nader argues that when he pushed Razieh, it was implausible she could have fallen down the stairs as the railing would have kept her safe. Meanwhile, Razieh confides in Simin that she had been struck by a car prior to the incident, adding complexity to the narrative of blame and injury.
Hodjat's violent tendencies threaten Termeh's safety, causing Simin to urge Nader to settle matters with him financially. Nader presses Razieh to solemnly swear on the Qur'an that he caused her miscarriage, a demand she cannot meet. Hodjat, unable to compel her to act, becomes increasingly volatile, resorting to self-harm in his rage.
Ultimately, Nader and Simin are granted a divorce in family court. The film concludes with Nader and Simin waiting independently outside the court as Termeh prepares to tell the judge whom she wishes to live with, encapsulating the emotional turmoil and broken familial bonds that have unfolded.
